Smartphones and tablets are an indispensable part of modern communication and people spend considerable time interacting with their devices every day. While substantial research has been conducted concerning smartphone usage, little is known about how tablets are used. This paper studies mobile device usage characteristics like session length, interaction frequency, and daily usage in locked and unlocked state with respect to location context. Based on logs from 1,585 Android devices (470 years of total usage time), we derive and analyze 23 million usage sessions. We found that devices remain locked for 60% of the interactions and usage at home occurs twice as frequent as at work. With an average of 58 interactions per day, smartphones are used twice as often as tablets, while tablet sessions are 2.5 times longer, resulting in almost equal aggregated daily usage. We conclude that usage session characteristics differ considerably between tablets and smartphones.